Media captionEd Miliband: "The circumstances we face will be more difficult than at any time for generations"

Labour would not increase borrowing to reverse cuts in government spending, Ed Miliband has told activists as he warned the party must be "credible".

Mr Miliband said Labour must face up to the "hard reality" that it will not be able to reverse spending cuts scheduled by the coalition for 2015-16.

The party's leader told its policy forum if Labour wins the next election, it will be in "tough" economic times.

The Conservatives said Mr Miliband was "too weak" to stick to the promise.

Mr Miliband said: "Our starting point for 2015/16 is that we won't be able to reverse the cuts in day-to-day current spending unless it is fully funded from savings elsewhere or extra revenue, not from more borrowing."

'Crystal clear'

He was speaking ahead of Chancellor George Osborne's Spending Review on Wednesday, which will set out the government's spending plans for the year following the general election in 2015.

Analysis

By Alan SoadyPolitical reporter, BBC News

Ed Miliband is attempting a balancing act.

On the one hand, he wants to enthuse his party by promising a Miliband government would introduce radical social changes; on the other, he's trying to convince voters that those changes wouldn't involve borrowing more money - at least in the first year.

He wants to be seen as trustworthy and disciplined on the economy, but he risks giving some voters the impression that he's not offering much that's different from the coalition parties.

His nostalgic references to the Attlee government are an attempt to show he has a bold vision.

But as the general election draws closer, he'll come under increasing pressure to translate that vision into hard, detailed policies.

And that's where his critics claim he'll come unstuck.

The majority of government departments are facing cuts of between 8% and 10%.

"When George Osborne stands up next week and announces his cuts..." Mr Miliband said, "we won't be able to promise now to reverse them because we've got to be absolutely crystal clear about where the money is coming from."

Hammering home his message on the economic inheritance Labour will face should it regain power after the next election, Mr Miliband described it as the "toughest for a generation".

Labour has already said it would withdraw the winter fuel allowance from the wealthiest pensioners, and would not reverse coalition cuts to child benefit for households where one person earns more than £50,000.

"People will only put their trust in us if we show we are credible," Mr Miliband said.

'Same old Labour'

But the Conservative Party chairman, Grant Shapps, dismissed the pledge, saying the opposition leader was "too weak to stick to his promises".

"Ed Miliband only offers more spending, more borrowing and more debt - the same old Labour approach that got us into this mess in the first place," he said.

Mr Miliband also set out plans to penalise property developers who "sit on land" rather than building new homes.

And he drew parallels with Clement Atlee's 1945 Labour government, saying that as well as creating the NHS and building homes, it also ran budget surpluses.

"By the end of their time in government they were actually starting to pay off the war time debt. But nobody in this party and nobody in this country thinks that government didn't make a difference," he said.

In an earlier speech to Liberal Democrat activists, leader Nick Clegg warned his party it risks "irrelevance" if it retreats to the "comfort blanket of opposition".